The present invention relates generally to the superfinishing of components manufactured from alloys containing high density carbides.
Contacting components of working machines are made from steel alloys and operate under loading. Eventually the contacting components experience wear and/or fatigue leading ultimately to equipment failure. Examples of contacting components are gears, crankshaft, camshafts, tappets, lifters, bearing rollers, races or cages, or similar components. It is often desired to harden the contact surface of such components to the highest hardness possible in order to reduce wear and to increase equipment life. Examples of contact surface hardening techniques are heat treatments, ion implantation treatments, and additive engineered coating treatments such as diamond like carbon. Contact surface hardening is especially desired for equipment operating under very high loading such as large power train systems including off-highway equipment such as bull dozers, dump trucks and mining equipment, marine systems such as tug boats and ferries, and power generation systems such as gas turbine generators and wind turbine generators. Although extensive effort has been carried out over the years by large power train system manufacturers to increase the contact surface hardness of working components, smaller power train system manufacturers, such as commercial automobile manufacturers, have also shown equal interest in achieving higher hardness contact surface working components.
Similarly, extensive efforts has been carried out over the years by other industries to increase the surface hardness of metal alloys for use in other working components that require high surface durability on their contact surfaces, such as for biomedical implants, cutting tools, punches, dies, extrusion tools, expansion tools and the like.
Numerous alloys and heat treatment methods have been developed, evaluated and selected to achieve this goal. For example, U.S. Pat. No. 4,921,025, “Carburized Low Silicon Steel Article and Process,” teaches a process for forming carburized steel articles containing not more than 1.1% chromium to form an austenitic surface matrix having a high density of carbides dispersed therein. After quenching, the carburized steel article is characterized by an outer surface having a high ratio of carbides and is substantially free of intergranular oxides. Components such as gears, shafts, bearings and couplings made from such carburizing treatment are greatly enhanced with regards to bending fatigue strength, wear properties, and contact fatigue strength. U.S. Pat. No. 5,910,223, “Steel Article Having High Hardness and Improved Toughness and Process for Forming the Article,” teaches a process for producing articles from alloys such as SAE 4122 having a surface of high density carbides of approximately 20% of the quantifiable area.
High hardness components generally require the highest quality of contact surface finishes in order to achieve their operational performance potential. Typically, the component manufacturer will require high quality contact surface finishes of Ra less than 0.25 micron or better, which are considered superfinishes. For high hardness contact surfaces, conventional grinding, honing, lapping or other surface finishing techniques becomes more and more difficult. Tool wear, for example, is accelerated as the hardness of a component is increased. Grinding, honing, lapping and the like must also be done with increasingly greater care as hardness increases in order to prevent “grind burn”. Grind burn is harmful since it softens the contact surface resulting in premature wear and component failure. Furthermore, the high hardness of these components, coupled with the difficulties associated with conventional grinding, honing, lapping and the like, make it difficult to maintain the dimensional geometry of the components. Thus, high hardness components finished by conventional grinding, honing, lapping and the like must often undergo a 100% final inspection to ensure component integrity.
Even if extremely hard contact surfaces can be superfinished via grinding, honing, lapping and the like, peak to valley asperities still remain on the contact surface and cause performance problems. These residual asperities are monotropic in orientation which are not ideal for lubrication. Also, under high loading, even small peaks to valleys penetrate the lubricating film resulting in metal-to-metal contact. It is well known in the art that metal-to-metal contact between contacting components where one or both of the contact surfaces have a high hardness is more damaging than for components having lower hardnesses. This is true because components having lower hardnesses will rapidly wear off the peak to valley asperities leaving a relatively smooth contact surface with the asperities leveled. In fact, this peak to valley asperity leveling is often done under light loading during a “break-in” or “run-in” cycle prior to subjecting the equipment to full loading. By contrast, where one or both contact surfaces are made from high density carbide material, the peak to valley asperities will be fractured from the contact surface as metal-to-metal contact occurs under high loading. Such an occurrence will produce wear, stress risers and distressed metal that are initiation sites for future fatigue failure. Additionally, where one of the mating contact surfaces is made of high density carbide material. The peak to valley asperities from the high density carbide contact surface will micro-cut or micro-plow the softer mating contact surface, thereby resulting in accelerated wear, production of stress risers, and loss of contact surface geometry.
Concomitant with wear is the generation of metal debris. Metal debris from high density carbide hardened contact surfaces is more damaging than debris from softer contact surfaces. Metal debris not only damages the components from which they are generated, but also other critical components such as bearings even when lubricant filtration systems are in place. The above discussion is emphasized in U.S. Pat. No. 6,217,415 B1, “Method and Arrangement for Reducing Friction Between Metallic Components,” which discusses how the rate of scuffing, wear, or pitting on the contact surface is the result of friction between the contact surface of the work machine component and a contacting surface of another work machine component. The inventor further discusses that mechanical polishing has been utilized to decrease friction between the contacting surfaces of work machine components, however, it is stressed that even after extensive mechanical polishing, microscopic contact surface irregularities (i.e., asperities) will still be present on the contacting surfaces of the work machine components. Therefore, even after mechanical polishing, there is a significant amount of friction between the contacting surfaces of work machine components due to the remaining asperities.
To eliminate the problems associated with conventional mechanical machining to reduce the contact surface roughness of high hardness contacting components, chemically accelerated vibratory finishing has been tested and evaluated. One benefit of chemically accelerated vibratory finishing over conventional machining is that it levels the peak to valley asperities. U.S. Pat. No. 4,491,500, “Method for Refinement of Metal”, and U.S. Pat. No. 4,418,333, “Metal Surface Refinement Using Dense Alumina-Based Media,” both of which are incorporated by reference in their entireties herein, teach the use of chemically accelerated vibratory finishing to superfinish hardened metal workpieces. The equipment can consist of a finishing barrel, vibratory bowl or a vibratory tub, centrifugal disc machine, drag finishing machine, plunge finishing machine or spindle finishing machine and the like. U.S. Pat. No. 6,656,293 B2, “Surface Treatment for Ferrous Components,” teaches the advantage of isotropic finishing nitrided or nitrocarburized metal to a surface roughness with an Ra less than 0.05 μm using chemically accelerated vibratory finishing. U.S. Pat. No. 5,503,481, “Bearing Steels with Isotropic Finishes,” applies the teaching of U.S. Pat. No. 4,491,500 and U.S. Pat. No. 4,418,333 to superfinish hardened steel bearings.
Prior to the present invention, attempts were made to superfinish these hard contact surfaces using chemically accelerated vibratory finishing. FIG. 1 is a diagrammatic cross-section through a machined surface layer 2 containing high density carbides 1 below which is the basis metal 4. As previously discussed, chemically accelerated vibratory finishing typically levels the peak 3 to valley 9 asperities that were produced in the mechanical machining process leaving a relatively flat surface. However, prior attempts at chemically accelerated vibratory finishing produced an undesirable contact surface 2 as shown in FIG. 2. FIG. 2 illustrates one possible outcome of an attempt using chemically accelerated vibratory finishing on contact surface 2 containing high density carbides, where the carbide particles 5 protrude from the contact surface 2. This is a highly undesirable contact surface since the carbide particles 5 can penetrate the lubricating film similarly to peak to valley asperities, thereby resulting in premature wear. Another serious problem with such a contact surface is that the carbide particles 5 can easily be dislodged from the contact surface resulting in highly damaging metal debris. FIG. 3 illustrates another undesirable outcome using chemically accelerated vibratory finishing. FIG. 3 illustrates that although the high density carbide particles 6 might be partially leveled, the metal surrounding the carbides has dissolved away leaving a weakened contact surface structure 7, which will fail under high loading and quickly disintegrate leading to high wear and metal debris.
It is desirable to harden the contact surface of contacting components to as high a hardness as possible in order to reduce wear and increase equipment life. Components manufactured from alloys such as SAE 4122 having a contact surface of high density carbides of approximately 20% of the quantifiable area have these desired high hardness properties. As discussed above, conventional machining is impractical and still leaves peak to valley asperities that have a negative impact under loading. Attempts at using chemically accelerated vibratory finishing based on the prior art have failed, and created contact surfaces with highly undesirable properties—either carbide particles protrude from the contact surface, or the metal supporting the carbides is dissolved away leaving a weakened contact surface structure. What is needed is a commercially practical and successful method for superfinishing components having a contact surface layer containing high density carbides.